Help me create fog for my movie
Best solution that doesn't take incredibly long to render is to use the Mist pass and composite in the fog. On the View Layers tab turn on Mist.



Then, in the compositor, turn on Use Nodes and add a Mix node between the Render Layers node and the Composite Node. Plug the Mist output into the Fac input on the mix node. The bottom slot controls your mist color, but you can also plug any other texture into this. You could for instance in another scene create a flat plane with an emission shader and a noise texture added to it and animate the texture node z position. This would look like clouds moving towards you. Then plug this animated texture into the bottom image slot the Mix shader in your main scene.



The falloff of the Mist is controlled in the Mist Pass panel in the World tab. Depth controls how far into the mist you can see. I would animate this to simulate the effect of changing fog density as you move across the landscape. Basically the thicker the cloud, the lower the depth.